As is known, there are essentially two types of electrostatic precipitators. In one, called a single-stage precipitator, particles entrained in a gas stream are charged in passing through a corona discharge and are then collected on grounded electrodes disposed adjacent the emitter electrodes which produce the corona discharge. In the other type of precipitator, called a two-stage precipitator, the particles are initially charged by a corona discharge and then travel downstream to collecting plates. In either case, a wire at high potential is mounted midway between relatively large electrodes. The high electric field at the wire produces a glow which is the source of ions. If the wire is negative, negative ions will be repelled from the wire and the ions will travel through the gas toward the passive or grounded electrode. Dust is charged by passing through this corona discharge, and some of this dust will be deposited on the passive electrode, even in the case of a two-stage precipitator. The corona current must then be conducted through this layer of collected dust; and even though the corona current is only a fraction of a microampere per square centimeter, if the dust has a high resistivity, the voltage drop through the dust may exceed its breakdown voltage gradient. This gives high local electric fields at the dust surface which produce the well-known back-corona effect wherein an electrical breakdown of the dust layer occurs at one or more points and, for example with negative corona, results in positive ions which partly neutralize the negative charge which the dust particles received in the ionizing corona. This back-corona effect greatly reduces the particle charge and may reduce the voltage which can be applied to the ion-emitting electrode or wire.